Hmm, it *sounds* like you have a vacuum leak and your vacuum pump continues to run, trying to get proper pressure to unlock the doors. I say this because you say it's coming from the trunk area, and happens right after you unlock the doors...
I would get in the car, be very quiet, and lock, then unlocking the doors, then try and find a hissing sound (like air rushing) to see which of the door actuators might be leaking. Are you having any other problems with your door locks? One not unlocking, persay, etc., etc.
Good luck!